High-frequency measurement method and high-frequency measurement apparatus

ABSTRACT

With a conventional high-frequency measurement method, it is difficult to accurately grasp variation in high-frequency performance when a high-frequency signal is input to an amplifier. One aspect of a high-frequency measurement method according to the present invention includes generating a test signal (TS), which is a sine-wave signal having a predetermined frequency, in which a period (τ) during which the power level is at a first power level and a period (T-τ) during which the power level is at a second power level lower than the first power level are periodically repeated, inputting the test signal (TS) to a device under test ( 10 ) as an input signal, and measuring the difference between an output signal (OUT) of the device under test ( 10 ) and an ideal value of the output signal (OUT).

TECHNICAL FIELD

The present invention relates to a high-frequency measurement method anda high-frequency measurement apparatus, and relates to, for example, ahigh-frequency measurement method and a high-frequency measurementapparatus for measuring an amplifier that amplifies a high-frequencysignal.

BACKGROUND ART

In a mobile broadband communication represented by mobile phones, it isrequired to transmit a broadband signal with the bandwidth of a few MHzto tens of MHz having a large peak-to-average power ratio (PAPR) withlow distortion. Especially with regard to a downlink signal to be sentfrom a base station, an amplifier is generally operated at a possiblehighest level using a distortion compensation function, such as digitalpredistortion, while back-off corresponding to the signal PAPR issecured, in order to reduce the power consumption of a transmissionamplifier while the adjacent channel leakage ratio defined in the 3GPPstandards is satisfied.

For recent amplifiers, transistors using a compound semiconductor (for,example, gallium nitride (GaN)) that achieves miniaturization and highefficiency as amplifying elements have been used. It is known thatcurrent collapse occurs when a high electric field is applied to a GaNtransistor. In current collapse, charges are trapped in the surface ofor inside the semiconductor, and the current lowers. Current collapsehas a time response in which the current collapse is recovered overtime. Such a time response means that the operation point of atransistor changes over time, and the characteristics of an amplifier,such as the power gain or nonlinearity, vary over time when thetransistor is used for the amplifier. This variation continues forhundreds of nanoseconds to seconds.

On the other hand, digital predistortion operates at a speed from tensof nanoseconds to microseconds in order to monitor a band which isseveral times wider than a signal band. This time has an equivalentorder of the recovery time of current collapse, and this means that thecharacteristics of the amplifier vary every moment while the digitalpredistortion is operated. In addition, the variation amount of theamplifier characteristics due to current collapse varies depending on anapplied high-electric field and time, and is not uniform. As a result,the digital predistortion cannot follow, and the distortion compensationfunction can be degraded. Since the amount of current collapse occurringwhen a high-electric field is applied to a transistor and the variationof the amplifier performance due to the current collapse vary dependingon individual transistors, it is possible to evaluate the compatibilitywith digital predistortion if the variation amount is measured.

Patent Literature 1 discloses an example of a semiconductor inspectionapparatus that evaluates current collapse. The semiconductor inspectionapparatus disclosed in Patent Literature 1 includes a first detectingunit electrically connected to a source electrode of a field-effecttransistor, a first diode including a first cathode electrodeelectrically connected to a drain electrode of the field-effecttransistor, a second detecting unit electrically connected to a firstanode electrode of the first diode, a first resistance element havingone end connected to the first anode electrode, and a first electricpower source connected to the other end of the first resistance element.

CITATION LIST Patent Literature

Patent Literature 1: Japanese Unexamined Patent Application PublicationNo. 2016-57091

SUMMARY OF INVENTION Technical Problem

However, since the semiconductor inspection apparatus disclosed inPatent Literature 1 supplies a direct-current signal to a field-effecttransistor, which is a device under test, it is difficult to accuratelygrasp variation in high-frequency performance when a high-frequencysignal is input to an amplifier.

Solution to Problem

One aspect of a high-frequency measurement method according to thepresent invention is a high-frequency measurement method for measuringan amplifier or a semiconductor amplifying element as a device undertest, the high-frequency measurement method including generating a testsignal, the test signal being a sine-wave signal having a predeterminedfrequency, in which a period during which a power level is at a firstpower level and a period during which the power level is at a secondpower level lower than the first power level are periodically repeated,inputting the test signal to the device under test as an input signal,and measuring a difference between an output signal of the device undertest and an ideal value of the output signal.

Another aspect of the high-frequency measurement method according to thepresent invention is a high-frequency measurement method for measuring acompound semiconductor transistor as a device under test, thehigh-frequency measurement method including generating a test signal,the test signal being a sine-wave signal having a predeterminedfrequency, in which a period during which a power level is at a firstpower level and a period during which the power level is at a secondpower level lower than the first power level are periodically repeated,inputting the test signal to the device under test as an input signal,and measuring, based on an output signal of the device under test, atransient response in gain variation of the device under test.

One aspect of a high-frequency measurement apparatus according to thepresent invention includes a signal generator that outputs a testsignal, the test signal being a sine-wave signal having a predeterminedfrequency, in which a period during which a power level is at a firstpower level and a period during which the power level is at a secondpower level lower than the first power level are periodically repeated,and a measuring instrument that measures a difference between an outputsignal of a device under test and an ideal value of the output signal.

Advantageous Effects of Invention

With a high-frequency measurement method and a high-frequencymeasurement apparatus according to the present invention, it is possibleto accurately grasp variation in high-frequency performance when ahigh-frequency signal is input to an amplifier.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing a high-frequency measurement apparatusaccording to a first example embodiment.

FIG. 2 is a diagram explaining a test signal of the high-frequencymeasurement apparatus according to the first example embodiment.

FIG. 3 is a diagram explaining a time response of an output signal of adevice under test measured by the high-frequency measurement apparatusaccording to the first example embodiment.

FIG. 4 is a block diagram showing a high-frequency measurement apparatusaccording to a second example embodiment.

FIG. 5 is a block diagram showing a high-frequency measurement apparatusaccording to a third example embodiment.

FIG. 6 is a block diagram showing a high-frequency measurement apparatusaccording to a fourth example embodiment.

DESCRIPTION OF EXAMPLE EMBODIMENTS First Example Embodiment

Hereinafter, example embodiments of the present invention are describedwith reference to the drawings. For the sake of clarify, the followingdescriptions and drawings are omitted and simplified as appropriate. Inthe drawings, the same elements are denoted by the same reference signs,and duplicated descriptions are omitted as necessary.

First, a high-frequency measurement apparatus 1 according to a firstexample embodiment measures, for example, a compound semiconductortransistor, such as gallium nitride (GaN) transistor, or an amplifierincluding a compound semiconductor transistor as a device under test.

FIG. 1 is a block diagram showing the high-frequency measurementapparatus 1 according to the first example embodiment. As shown in FIG.1, the high-frequency measurement apparatus 1 according to the firstexample embodiment measures a device under test 10. The high-frequencymeasurement apparatus 1 according to the first example embodimentincludes a signal generator 20 and a measuring instrument 30.

The signal generator 20 generates a test signal TS, which is a sine-wavesignal having a predetermined frequency, in which a period during whicha power level is at a first power level and a period during which thepower level is at a second power level lower than the first power levelare periodically repeated.

The signal generator 20 includes, as an example, a first signalgenerator 21, a second signal generator 22, and a combiner 23.

The first signal generator 21 outputs a first sine-wave signal PS1having the first power level. The second signal generator 22 outputs asecond sine-wave signal PS2 having the second power level. The combiner23 combines the first sine-wave signal PS1 and the second sine-wavesignal PS2 to output the test signal TS. Note that, in the case ofconsidering the power loss at the combiner 23, the power levels of thefirst sine-wave signal PS1 output from the first signal generator 21 andof the second sine-wave signal output from the second signal generator22 are assumed to be suitable power levels considering the power loss.

The measuring instrument 30 measures the difference between an outputsignal OUT of the device under test 10 and an ideal value of the outputsignal OUT. Specifically, the measuring instrument 30 measures thedifference between the output signal OUT relating to a time response orfrequency response of the output signal OUT and the ideal value. Thetime response of the output signal OUT can be a transient response ofthe power waveform of the output signal OUT, a signal distortion of theoutput signal OUT, a transmission delay, or the like. The frequencyresponse of the output signal OUT can be a phase rotation amount of theoutput signal OUT, or the like. In the following, the high-frequencymeasurement apparatus 1 is described on the assumption that themeasuring instrument 30 measures a transient response of the powerwaveform of the output signal OUT.

Next, an operation of the high-frequency measurement apparatus 1according to the first example embodiment is described. First, the testsignal TS is described. FIG. 2 is a diagram explaining a test signal ofthe high-frequency measurement apparatus according to the first exampleembodiment.

As shown in FIG. 2, the test signal TS is generated by combining thefirst sine-wave signal PS1 and the second sine-wave signal PS2. Thefirst sine-wave signal PS1 is a signal in which a period having a sinewave at the first power level and a period having no signal are repeatedin a cycle T. In the first sine-wave signal PS1, a period τ having thesine wave is about from tens of nanoseconds to milliseconds that is atime having an equivalent order to that of the sampling interval or thesignal frame length of a modulated wave signal used by mobile phone basestations or the like. In addition, the frequency of the sine wave of thefirst sine-wave signal PS1 is about from 800 MHz to 3.5 GHz that is acarrier frequency of a signal used by mobile phone base stations or thelike, or is a higher frequency including a millimeter wave band. Thesecond sine-wave signal PS2 is a signal having a continuous sine wave atthe second power level. Note that, the second power level issignificantly small power compared to the first power level and isassumed to have the power difference that can be approximated as thefirst power level in the period having two sine waves when the firstsine-wave signal PS1 and the second sine-wave signal PS2 are combined.

By combining the first sine-wave signal PS1 and the second sine-wavesignal PS2, it is possible to generate the test signal TS in which theperiod T-τ during which the power level is at the first power level, theperiod T-τ after period τ passes during which the power level is at thesecond power level, and the first-power-level period and thesecond-power-level period are repeated in the cycle T. FIG. 2 also showsa graph showing the test signal TS expressed by the power level.

In addition, the length of the period T-τ is preferably set to thelength of time required to observe the recovery of current collapsehaving occurred in the device under test 10, and is, for example, aboutfrom hundreds of nanoseconds to seconds.

When the test signal TS shown in FIG. 2 is input to an amplifier or asemiconductor amplifying element, which is the device under test 10,current collapse is induced in the device under test 10 in thefirst-power-level period. Then, immediately after the power level of thetest signal TS reaches the second power level, the current collapserecovers gradually. Thus, a time response of the output signal OUT ofthe device under test 10 when current collapse occurs and recovers isdescribed. FIG. 3 shows a diagram explaining a time response of anoutput signal of a device under test measured by the high-frequencymeasurement apparatus according to the first example embodiment. In FIG.3, a first output power level is the power output from the device undertest 10 when the input signal level is at the first power level shown inFIG. 2. In FIG. 3, a second output power level is the power output fromthe device under test 10 when the input signal level is at the secondpower level shown in FIG. 2 and when current collapse does not occur inthe device under test 10.

As shown in FIG. 3, in the power waveform of the output signal OUTobtained by input the test signal TS to the device under test 10measured by the high-frequency measurement apparatus 1 according to thefirst example embodiment, the power of the output signal OUT becomeslower than the second output power level when the power level of theinput signal is changed from the first power level to the second powerlevel, and, then, gradually returns to the second output power level.The reference “A” in FIG. 3 shows the waveform while current collapse isrecovering.

The lower part of FIG. 3 shows the enlarged power waveform of the outputsignal OUT during the period A. As shown in the lower part of FIG. 3,the change in the power waveform due to the recovery of the currentcollapse can be expressed by a time dT and a power difference dP. Themeasuring instrument 30 measures the time dT and the power differencedP. The measuring instrument 30 can use three measurement methods. Afirst measurement method is to measure the time dT until the power levelof the output signal OUT returns the second output power level and achange amount dP of the power level of the output signal OUT. A secondmeasurement method is to measure the time dT until the power level ofthe output signal OUT reaches a certain change amount dP. A thirdmeasurement method is to measure a change amount dP of the power levelof the output signal OUT at a time when a certain time dT passes sincethe input signal is changed from the first power level to the secondpower level.

Note that, the power waveform of the output signal OUT when currentcollapse is recovered is not limited to a linear change (the solid linein FIG. 3), and can be a nonlinear change as shown by thedashed-and-dotted line or the dashed-and-double-dotted line in FIG. 3,or an undulating change.

As described above, in the high-frequency measurement apparatus 1 andthe high-frequency measurement method according to the first exampleembodiment, the test signal TS simulating a signal to be input when thedevice under test 10 is actually operated is generated, and the gainvariation of the device under test 10 due to current collapse ismeasured based on the test signal TS. Thus, with the high-frequencymeasurement apparatus 1 and the high-frequency measurement methodaccording to the first example embodiment, it is possible to performquantitative evaluation of current collapse in the device under test 10with high accuracy.

In addition, the influence of current collapse differs depending on themagnitude and time of a high-electric field applied to the device undertest 10. Thus, by adjusting the first power level and the time τ so asto meet the operating conditions, it is possible for the high-frequencymeasurement apparatus 1 according to the first example embodiment tomeasure and evaluate the influence on amplifier characteristics due tocurrent collapse during the operating state.

Furthermore, it is known that phase characteristics, delaycharacteristics, or distortion characteristics are changed in currentcollapse. With regard to these characteristics, by changing items to bemeasured by the measuring instrument 30, it is possible for thehigh-frequency measurement apparatus 1 according to the first exampleembodiment to measure and evaluate the change in these characteristicsin a state close to the operating state.

Note that, the above description has been made on the assumption ofcurrent collapse in a GaN semiconductor in which charges are trapped bya high-electric field, but the measurement method according to thepresent invention is applicable to, for example, measurement orevaluation of the influence of a phenomenon called drain lag or gate lagin other semiconductor materials, such as GaAs, on high-frequencycharacteristics.

Second Example embodiment

In a second example embodiment, a combiner 23 a, which is a specificexample of the combiner 23, is described. FIG. 4 is a block diagramshowing a high-frequency measurement apparatus 2 according to the secondexample embodiment. As shown in FIG. 4, the combiner 23 a is adirectional coupler. The combiner 23 a transmits a first sine-wavesignal PSI through an input/output path and inputs a second sine-wavesignal PS2 to a coupling terminal.

In the relationship between the first power level and the second powerlevel, the first power level is larger than the second power level.Thus, by transmitting the first sine-wave signal PSI using aninput/output path having low transmission loss at the directionalcoupler, it is possible to easily achieve a time response shown in FIG.2. In addition, the difference between the first power level and thesecond power level can be about 20 dB to 30 dB, and it is also possibleto achieve such the level difference with the coupling amount of thedirectional coupler.

Third Example embodiment

In a third example embodiment, a signal generator 20 b, which is adifferent example embodiment from the signal generator 20, is described.FIG. 5 is a block diagram showing a high-frequency measurement apparatus3 according to the third example embodiment. As shown in FIG. 5, thesignal generator 20 b is configured by adding a first attenuator 24 anda second attenuator 25 to the signal generator 20.

The first attenuator 24 is provided between a first signal generator 21and a combiner 23. The first attenuator 24 adjusts the amplitude of afirst sine-wave signal PS1. The second attenuator 25 is provided betweena second signal generator 22 and the combiner 23. The second attenuator25 adjusts the amplitude of a second sine-wave signal PS2.

Since current collapse depends on the magnitude and time of an electricfield to be applied, a device under test under various conditions can bemeasured as long as the first power level is varied. Furthermore, thefirst power level is required to be sufficiently high to drive thedevice under test in a nonlinear region in which the power gain of thedevice under test is suppressed compared to the linear gain. Thus, byadjusting the first power level with the first attenuator 24, it ispossible to easily adjust a higher power level required for anyamplifier. Moreover, the characteristic after current collapse occurs,that is, the response in the period A shown in FIG. 3 depends on thesecond signal level. For these reasons, by varying the second powerlevel with the second attenuator 25, it is possible to measure thebehavior of the device under test 10 under various conditions.

Fourth Example Embodiment

In a fourth example embodiment, a high-frequency measurement apparatus4, which is a different example embodiment of the high-frequencymeasurement apparatus 1, is described. FIG. 6 is a block diagram showinga high-frequency measurement apparatus 4 according to the fourth exampleembodiment. As shown in FIG. 6, the high-frequency measurement apparatus4 according to the fourth example embodiment includes a measuringinstrument 30 a instead of the measuring instrument 30.

The measuring instrument 30 a uses a test signal TS, which is an inputsignal of the device under test 10, as a waveform of an ideal value ofan output signal OUT. By generating the ideal value of the output signalOUT using a signal to be input to the device under test 10, it ispossible to shorten the measuring time without setting an ideal valuedepending on a change in the conditions of the test signal TS.

The present invention has been described with the above exampleembodiments, but is not limited by the above example embodiments.Various modifications that can be understood by those skilled in the artcan be made to the configurations and the details of the presentinvention without departing from the scope of the invention.

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2017-021155, filed on Feb. 8, 2017, thedisclosure of which is incorporated herein in its entirety by reference.

REFERENCE SIGNS LIST

-   1 High-frequency measurement apparatus-   2 High-frequency measurement apparatus-   3 High-frequency measurement apparatus-   4 High-frequency measurement apparatus-   10 Device under test-   20 Signal generator-   21 First signal generator-   22 Second signal generator-   23 Combiner-   24 First attenuator-   25 Second attenuator-   30 Measuring instrument-   PS1 First sine-wave signal-   PS2 Second sine-wave signal-   TS Test signal-   OUT Output signal

1. A high-frequency measurement method for measuring an amplifier or asemiconductor amplifying element as a device under test, thehigh-frequency measurement method comprising: generating a test signal,the test signal being a sine-wave signal having a predeterminedfrequency, in which a period during which a power level is at a firstpower level and a period during which the power level is at a secondpower level lower than the first power level are periodically repeated;inputting the test signal to the device under test as an input signal;and measuring a difference between an output signal of the device undertest and an ideal value of the output signal.
 2. The high-frequencymeasurement method according to claim 1, wherein a period during whichthe test signal maintains the first power level is set to a lengthequivalent to a period during which an input signal intended for thedevice under test is at the first power level.
 3. The high-frequencymeasurement method according to claim 1, wherein the difference is adifference relating to a time response or a frequency response of theoutput signal.
 4. The high-frequency measurement method according toclaim 1, wherein the difference is a difference between the outputsignal and the ideal value relating to at least one of a power waveform,a waveform distortion, a phase rotation amount, and a transmission delayof the output signal.
 5. The high-frequency measurement method accordingto claim 1, wherein the amplifier or the semiconductor amplifyingelement is composed of a compound semiconductor transistor.
 6. Thehigh-frequency measurement method according to claim 1, wherein thefirst power level is higher than the second power level.
 7. Thehigh-frequency measurement method according to claim 1, wherein theideal value is generated based on the test signal.
 8. A high-frequencymeasurement method for measuring a compound semiconductor transistor asa device under test, the high-frequency measurement method comprising:generating a test signal, the test signal being a sine-wave signalhaving a predetermined frequency, in which a period during which a powerlevel is at a first power level and a period during which the powerlevel is at a second power level lower than the first power level areperiodically repeated; inputting the test signal to the device undertest as an input signal; and measuring, based on an output signal of thedevice under test, a transient response in gain variation of the deviceunder test.
 9. A high-frequency measurement apparatus comprising: asignal generator configured to output a test signal, the test signalbeing a sine-wave signal having a predetermined frequency, in which aperiod during which a power level is at a first power level and a periodduring which the power level is at a second power level lower than thefirst power level are periodically repeated; and a measuring instrumentconfigured to measure a difference between an output signal of a deviceunder test and an ideal value of the output signal.
 10. Thehigh-frequency measurement apparatus according to claim 9, wherein thesignal generator comprises: a first signal generator configured tooutput a first sine-wave signal having the first power level; a secondsignal generator configured to output a second sine-wave signal havingthe second power level; and a combiner configured to combine the firstsine-wave signal and the second sine-wave signal to output the testsignal.